This invention relates to rigid polyurethane or urethane-modified polyisocyanurate foams, to processes for their preparation and to polyol blends for use in said processes.
Rigid polyurethane and urethane-modified polyisocyanurate foams are in general prepared by reacting a stoichiometric excess of polyisocyanate with isocyanate-reactive compounds in the presence of blowing agents, surfactants and catalysts. One use of such foams is as a thermal insulation medium in, for example, buildings.
Polyether polyols or polyester polyols are generally used as isocyanate-reactive compounds. Polyester polyols impart excellent flame retardancy characteristics to the resulting polyurethane foams and can in some cases even be less expensive than polyether polyols.
Tertiary amines are generally used as catalyst in rigid polyurethane foam systems based on polyester polyols. A problem encountered when using tertiary amine catalysts in these polyester rigid foam systems is that a cross-linked mass is obtained at a time when the foam has not fully filled the cavity yet (for example, of a laminated building panel). This leads to dimensional stability problems due to density distribution problems and cell stretching of the obtained foam.
Therefore it is an object of the present invention to provide a process for making rigid polyurethane foams based on polyester polyols not showing the disadvantages mentioned above.
According to the present invention a process for making rigid polyurethane or urethane-modified polyisocyanurate foams is provided by reacting an organic polyisocyanate composition with an isocyanate-reactive composition comprising a polyester polyol in the presence of an amine catalyst (B) characterised in that the pKa of the conjugated ammoniumsalt of the amine (BH+) is less than 12, preferably less than 10, more preferably less than 8.
pKa=xe2x88x92log Ka=xe2x88x92log [B][H+]/[BH+]
An acceptable rise profile is obtained having a fast initial foam rise leading to a smooth processability resulting in a better density distribution, lower minimum stable density and fill weights and higher compression strength of the foam.